The present invention relates to a safety switch, and in particular a safety switch arrangement.
Safety switches are well known, and are typically used to prevent access to for example dangerous electromechanical machinery when that machinery is in operation. In a conventional arrangement the safety switch is mounted on a doorpost of a machinery guard, and an actuator for the safety switch is mounted on a corresponding door. When the door is closed the actuator engages with the safety switch, which in turn closes a set of electrical contacts which allow power to be supplied to the machinery. This arrangement ensures that power can only be supplied to the machinery when the guard door is shut. When the guard door is opened, the actuator disengages from the safety switch, thereby opening the electrical contacts and cutting off the supply of power to the machinery.
A typical safety switch comprises a housing, in which is provided a set of contacts fixed in position relative to the housing. An axially slideable plunger is mounted inside the housing, and is moveable relative to the housing. The plunger (or another plunger in contact with the plunger, for example a contact block plunger) is provided with another set of contacts. The plunger is biased towards a cam arrangement by a spring. The actuator mentioned above is arranged to engage with the cam arrangement.
In many safety switches, if the actuator is not engaged with the cam arrangement (i.e. if the actuator is not engaged with the safety switch), the cam arrangement is arranged to prevent the contacts on the plunger coming into contact with the contacts of the housing by preventing movement of the plunger (i.e. the plunger is kept in a first plunger position). By preventing the contacts from contacting one another, the switch cannot conduct electricity while the actuator is not engaged with the cam arrangement.
Bringing the actuator into engagement with the cam arrangement causes the cam arrangement to rotate, which in turn causes the plunger (which is biased toward the cam arrangement) to move into a notch provided in the cam arrangement. The plunger is then in a second plunger position. When the plunger moves into the notch, the contacts on the plunger are brought into contact with the contacts of the housing, allowing electricity to flow through the safety switch.
Before a safety switch is sold to a customer, it is often desirable to undertake certain tests on that safety switch, or on a safety switch similar to that safety switch (for example, a safety switch from the same batch, production run, series of safety switch, etc.) to validate the quality of construction and operability of the switch. For instance, it may be desirable for the manufacturer or supplier of the safety switch to ensure that the safety switch performs to a desired level, for example a level quoted in literature or marketing documentation. The tests may also be desirable to ensure that the safety switch is of a desired standard, and is, for example durable enough to be used in the conditions in which the safety switch is intended to be used. For instance, in some jurisdictions throughout the world, safety switches have to meet certain safety criteria (for example, safety standards set by governments or government related bodies) which these switches have to meet in order to be sold or used in that particular country.
The testing mentioned above may take one of a number of forms. For instance, an actuator may be repeatedly engaged with and disengaged from the safety switch to check that the safety switch remains operable during and after the repeated engagement and disengagement. For example, the actuator may be engaged with and disengaged from the safety switch a hundred thousand times to ensure that, when sold, thereby increasing confidence that the same or similar safety switch will not fail prematurely.
If the safety switch (or switches) in question pass the test, the safety switch can be sold with reasonable confidence that the switch is durable enough to withstand a desired or expected number of operations. However, once sold, there is no way of telling how many times the safety switch has been used. For instance, there is no way of telling how many times somebody has entered and left an enclosure incorporating such a safety switch, and therefore how many times an actuator has been engaged with and disengaged from the safety switch. More importantly, there is no way of telling whether the safety switch is being used or has been used to such an extent that it exceeds the level up to which it was initially tested. For example, if the safety switch was tested to ensure that it retained structural integrity and an operating state for 50,000 operations, once installed for use, there is no reasonable way of telling the number of uses of the switch by the end user. That is, for an exemplary switch having an asserted useable life of at least 50,000 uses, there is no reasonable means for assessing if the end-user of the safety switch has used it 25,000 times and is within the level of testing, or if the end-user has used it 100,000 times and exceeded the level of testing. Since the safety switch may not have been tested to the extent which the end-user has used the switch, there is no way of telling if the safety switch is as safe as was intended. Clearly this is undesirable.
It is therefore an object of the present invention to provide a safety switch arrangement which may overcome or substantially mitigate at least one disadvantage of the prior art, whether identified herein or elsewhere.